TOP > Special > The X-ray Observatory to Make the Violent Universe Visible

Q. Please give us an outline of the Monitor of All-sky X-ray Image project and its goals.

MAXI installed on the Exposed Facility of Kibo (Courtesy of NASA)

MAXI’s vision. Blue shows the view through the GSC camera, and yellow shows the view through the SSC camera. The arrow indicates the direction of the ISS’s flight.

The Monitor of All-sky X-ray Image - known as MAXI - is a large X-ray camera built to observe the high-energy celestial objects that emit X-rays. X-rays are generated when high-energy particles collide with various materials and/or from ultra-high-temperature gas and jets around peculiar objects such as black holes, neutron stars, supernovae, variable stars, and active galaxies. But because X-ray intensities of these astronomical objects are changing continuously, rapidly and randomly, it is very hard to predict how and when we should observe an interesting moment of these phenomena.
In addition, X-rays are difficult to observe on the Earth, because they are absorbed by the atmosphere. So in order to detect X-rays from astronomical objects, we have to go beyond Earth’s atmosphere, into space. That’s why we developed MAXI and installed it in the Exposed Facility of the Japanese Experiment Module, Kibo, which was attached to the International Space Station in July 2009.
MAXI has semi-circular arc-shaped slit X-ray cameras that allow it to scan the entire sky. In the 90 minutes it takes the ISS to orbit the earth, MAXI gets a 360° image of the entire sky, including more than 1,000 astronomical objects that emit X-rays. This allows us to observe X-ray-emitting objects outside our galaxy, which until now has been difficult to do with wide field X-ray cameras. With normal X-ray astronomical satellites and telescopes, which have a narrower field of view, we can only observe a dedicated astronomical object we have chosen to target. But MAXI has two "X-ray eyes": the Gas Slit Camera (GSC) and an X-ray slit CCD camera called the Solid-state Slit Camera (SSC). Together they are possible to detect these transients and variable phenomena in space wherever they occur above detection limit of them. MAXI is 10 times more sensitive than previous all-sky X-ray observatory. Furthermore, MAXI, for the first time, can cover hard to soft X-rays with two kinds of X-ray eyes. Additionally, since the ISS can supply basic resources such as power, attitude control and communication systems, we were able to install larger-scale equipment on Kibo than so far performed All Sky X-ray Observatories.
MAXI was installed on Kibo’s Exposed Facility, the JEM-EF, in July 2009 by astronaut Koichi Wakata. The JEM-EF is also carrying further two payloads to study the space environment and Earth’s upper-atmosphere, but at the moment, MAXI is its only payload for astronomical observation. It is the first astronomical observation instrument installed on the ISS, and it will continue to observe X-ray-emitting celestial objects for at least two years. Its data analysis and operations are conducting as a joint project of JAXA, RIKEN, Osaka University, Tokyo Institute of Technology, Aoyama Gakuin University, Nihon University, Kyoto University, Miyazaki University, and Chuo University.

X-ray nova in the constellation Sagittarius. The arrow points to the location where MAXI spotted the nova and tracked it as it formed. MAXI started watching the star a week before it went nova, and followed its transformation. In this series of images, it changes from blue to red as its brightness and radiation energy grows. Coincidentally, you can also see a binary neutron star to the nova’s upper left, which resumed its activities and brightened at the same time as the nova. (*3) All-sky image taken by the X-ray CCD camera (*4)

MAXI’s mission is mainly to observe transient and variable phenomena of peculiar objects such as black holes and neutron stars in the entire sky over an extended long period. About one year has passed since the observations started, but MAXI has already discovered new X-ray-emitting celestial objects and detected various violent variabilities of known objects. Its observations continue, and we are still analyzing the data, so I cannot give you the details on what we have found, but I can summarize some of our initial observations briefly.

1) MAXI set a remarkable record of taking only two months to complete an all-sky X-ray image. In November 2009, we released an all sky image of X-ray objects made from Gas Slit Camera observations. MAXI was the first in the world to obtain the all-sky image within two months of starting operation, and to release those images to the public. Since then, MAXI has provided a new set of the all-sky image every two months, and after 10 months of observations we can now recognize about 350 X-ray-emitting objects. As X-ray intensities of these objects are always variable, we are going to continue observations over time, and we plan to publish the world’s first catalogue of variable X-ray sources. (Image 1)
In addition, we have made an all-sky animation from our series of daily all-sky images. This animation shows how the objects on the Galactic plane transiently appear and then disappear, and often vary violently. (Image 2) 2) Observation of gamma-ray bursts MAXI has detected several gamma-ray bursts, whose majority is confirmed to be related to supernova explosions. Gamma-ray bursts are the enormous explosions that are thought to be the largest in the Universe, and many of these bursts occur billions-light years away. Their process of X-ray generation is insufficiently understood, and they remain a mystery. 3) Long-term observation of X-ray novae of black hole binaries Over more than seven months, we observed an X-ray nova of a black hole binary as it appeared near the Galactic center. X-ray nova appeared suddenly like a bright firework, but MAXI succeeded in detecting a rising phase of an X-ray nova before the peak of its burst and following its evolution until it has vanished. Bright X-ray novae with black holes are seen only once or twice per year in the entire sky, so these MAXI data are very valuable, and we have already published the paper on it. (Image 3) 4) X-ray pulsar observations A binary X-ray pulsar consists of a neutron star with a strong magnetic field and an ordinary star. The gases from the ordinary star flow into the neutron star, and the brightness alters depending on the amount of gas flow. MAXI observed these X-ray pulsars, and obtained unique data on the structure and evolution of these gases. Additionally, it obtained the X-ray pulsars that had been re-activated after several years, as well as a new, little-known phenomenon; i.e. X-ray pulsars that extremely brighten only for a few hours. 5) Flaring activities of active galaxies MAXI observed active galaxies with producing jets that make variable X-ray intensely, like enormous flares. We obtained valuable data that could explain the conditions under which active galaxies eject jets by simultaneous observation with radio and ultra-high energy gamma-rays. MAXI also detected the jets which were produced from micro-Quasars; i.e. binary active stars accompanying a black hole or a neutron star. 6) Soft X-ray all sky images observed by the X-ray CCD camera MAXI performed the first all-sky soft X-ray observation using the X-ray CCD camera whose CCDs are produced by Japanese company. This allowed us to confirm that there are many soft X-ray objects along the galactic plane, as well as to reveal the existence of high-temperature plasmas surrounding the solar system. If we continue our observation on how much and what kind of elements exist in the hot plasma - such as the distribution of oxygen and neon emission lines - we could get close to solving the mystery of the origin and evolution of the hot plasmas. (Image 4)

These observation results are still being analyzed, and I think there could be new discoveries coming. The world has great expectations for MAXI’s all-sky observations. We hope that MAXI will continue to encounter never before seen celestial bursts and rapid variable phenomena.